This invention relates to a pipestill heater and a method for controlling combustion in a pipestill heater, and more particularly to a pipestill heater including a convection section and a radiation section for preheating process fluid by means of exhaust gas discharged from combustion devices and a method for controlling combustion in such a pipestill heater.
A conventional pipestill heater is generally constructed in such a manner as shown in FIGS. 6(A) and 6(B). More particularly, the pipestill heater includes a casing 100 made of a steel plate in which a radiation section 101 and a convection section 102 are provided. The radiation section 101 includes a furnace floor 103 constituting a part of a furnace wall, on which a plurality of combustion devices 104 are mounted. Also, the radiation section 101 is provided therein with a heating tube path 105 adapted to be heated by radiative heat transfer from the combustion devices 104. The heating tube path 105 is typically formed of a plurality of vertically extending straight tubes 106 wherein each adjacent two of the tubes 106 are connected at each of upper and lower ends thereof in series to each other by means of a U-shaped tube. The convection section 102 is arranged on a downstream side of a discharge flow path for exhaust gas discharged from the radiation section 101 and provided therein with a preheating tube path 107 connected to the heating tube path 105. Process fluid flowing through the preheating tube path 107 is preheated by exhaust gas of an elevated temperature discharged from the combustion devices before it is fed to the heating tube path 105. The preheating tube path 107 is likewise formed of a plurality of straight tubes wherein each adjacent two of the tubes are connected at each of both ends thereof in series to each other by means of a U-shaped tube. Exhaust gas discharged from the convection section 102 is guided to an intermediate flow path through a gas discharge path 109 provided with an induced draft fan 108. Alternatively, the exhaust gas may be naturally discharged without the induced draft fan 108. The combustion devices 104 each are so constructed that combustion air is fed to a burner 104b through a duct 104a equipped with a flow control damper by means of a forced draft fan. Alternatively, feeding of the combustion air may be carried out by a natural draft action without using the forced draft fan.
Heat efficiency exhibited by the conventional pipestill heater thus constructed is generally limited to a level as low as 60 to 85%. In order to increase the heat efficiency to a level as high as 90% or more, it is required to arrange an oxidizing agent preheating unit for preheating an oxidizing agent such as combustion air or the like. Unfortunately, this not only requires a large space for the arrangement but causes a cost for construction of the preheating unit to be increased to a level as high as that for a heater body. This is likewise true of replacement of the preheating unit deteriorated in performance. Also, it is demanded to install an equipment for pollution prevention attendant on arrangement of the heater. However, any existing pipestill heater generally fails to have a sufficient space which permits such a pollution prevention equipment to be added thereto.